According to the passage, the railroad played a significant role in literature in all of the following kinds of books EXCEPT ______.

A．They have as much literary importance as the books written by Emerson, Thoreau, and Adams.
B．They are good examples of the effects industry and business had on the literature of the United States.
C．They contributed to the weakening of traditional values.
D．They are worth reading as sources of knowledge about the impact of railroads on life in the United States.